Conventionally, there has been well-known such a three-dimensional image capturing apparatus that captures images of a subject from different positions by employing a plurality of imaging elements, so as to measure a position (a distance) of the subject on the basis of a parallax between the images of the same subject by using parameters, such as positions of the imaging elements, a focal distance, etc. (refer to Patent Document 1-3).
In the three-dimensional image capturing apparatus abovementioned, the luminance signal Y derived from each of the wavelength signals (for instance, RGB signals, etc.) is employed for detecting the distance to the subject. Conventionally, the luminance signal Y has been found by mixing the wavelength signals, outputted form the imaging elements, at a fixed mixing ratio. For instance, with respect to the RGB signals, the luminance signal Y has been found by mixing the RGB signals according to the predetermined mixing ratio defined by the following Equation.Y=0.3×R+0.6×G+0.3×B 
Further, the three-dimensional image capturing apparatus, mentioned in the above, has been so designed that it is possible to focus the subject whose distance is in a range of several ten centimeters to infinitive, when a fixed focal lens is employed for measuring the distance to the subject. Still further, the three-dimensional image capturing apparatus has evaluated the MTF (Modulation Transfer Function) of the luminance signal Y, etc., to determine Whether or not the subject is currently in focus, and accordingly, the range finding accuracy of the subject has been depending on the abovementioned MTF. Since the luminance signal Y abovementioned includes a large amount of the G signal component, the MTF characteristic of the luminance signal Y is approximate to that of the G signal component.
[Patent Document 1]                Tokkaihei 7-244717 (Japanese Non-Examined Patent Publication)        
[Patent Document 2]                Tokkai 2000-186929 (Japanese Non-Examined Patent Publication)        
[Patent Document 3]                Tokkai 2000-347133 (Japanese Non-Examined Patent Publication)        
Generally speaking, however, a focal point of the light reflected from the subject and penetrated through a pan-focus lens varies depending on the wavelength of the light concerned. For instance, as shown in FIG. 7, when the light reflected from the subject is focused onto a focal surface by pan-focus lens 16, the position of the focal surface varies depending on the wavelength of the light, such as Red (R), Green (G) or Blue (B). Further, as shown in FIG. 8(a), FIG. 8(b) and FIG. 8(c), when the focal lengths of lenses 17a, 17b and 17c are set at a constant value, an optimum position at which the subject is in focus varies depending on the wavelength of the light. Concretely speaking, the Red light reflected from the subject, located at a farther position compared to the Green light, is in focus as shown in FIG. 8(c) and FIG. 8(b), while the blue light reflected from the subject, located at a nearer position compared to the Green light, is in focus as shown in FIG. 8(a) and FIG. 8(b).
Accordingly, when the subject is positioned at such a close range that is equal to or shorter than several tens centimeters, the MTF characteristic of the luminance signal Y that includes a large amount of Green light signal as aforementioned is deteriorated and resulted in a blurred image, though the high MTF characteristic can be acquired with respect to the Blue light signal. Further, when the subject is a distant view, the MTF characteristic of the luminance signal Y that includes a large amount of Green light signal is deteriorated as well, though the high MTF characteristic can be acquired with respect to the Red light signal. As abovementioned, there has been a problem that, sometimes depending on the position of the subject, it is impossible to acquire a high accuracy of the range finding, when the luminance signal Y, generated by mixing the wavelength signals at a fixed mixing ratio, is employed.